Sliding steps are used to facilitate people boarding and deboarding public transit vehicles. For that purpose, they have a foldout and retractable tread plate with a sturdy tread surface. The tread plate is usually accommodated inside a frame serving as a receptacle, which is firmly attached to the vehicle.
In principle, there are several approaches to determine forces which act upon the tread plate. On the one hand, there must be entrapment protection; on the other hand, the tread plate must not be retracted as long as persons are standing on it. For that reason, sliding steps are often equipped with sensors to recognize any top or lateral loads.
In known sliding steps, sensing of the tread plate is often accomplished by means of a tread contact mat and/or proximity switches. Since the required height of access systems is becoming increasingly lower, it is more and more difficult to accommodate the detection elements. In addition, these are relatively susceptible to malfunction and subject to a high degree of wear due to unavoidable factors such as mechanical vibrations, dirt, moisture, splash water or frost.
To solve this problem, there are approaches that use bending sensors or strain gauges in the access system or sliding step. In principle, several positions are suitable for attaching the strain gauges. The main problem is that displacement occurs when the components or positions are under load. This displacement can be measured and evaluated as a signal with a strain gauge.
For example, a system is known from German Patent Publication No. DE 103 51 988 A1 in which the troublesome forces are detected in that the frame of the boarding/access arrangement is attached to the vehicle bottom with elastomer springs, thus allowing the frame to tilt by means of load cells comprising range springs and strain gauges. This requires a certain type of installation of the boarding/access arrangement to the vehicle. However, this is unsatisfactory due to the large number of different vehicle types. Furthermore, these known boarding/access arrangements have strain gauges which are arranged between the slideable tread plate and an associated guide of a crank drive and/or between a tilting and sliding tread plate and a carrier bar coupled therewith. This design is also very complex and has the disadvantage that complex towing cables or the like are necessary to connect the strain gauge to the necessary control units for the drive. This not only increases manufacturing costs, but is also connected with undesirable wear and tear, resulting in malfunction.
German Patent Publication No. DE 10 2009 039 035 A1 shows a tread frame and a tread plate linearly sliding therein. For detecting the forces applied to the tread plate, especially the vertical forces, at least one measuring element is provided which is arranged in the area of the at least one fastening element of a frame. According to an embodiment, the measuring element can contain a strain gauge such as a push-in type strain gauge. The installation and maintenance of the measuring element is relatively complex. In addition, the arrangement cannot easily be applied to other types of sliding steps without the need of technical modifications.
With the known systems, it is also often difficult to protect the strain gauges against mechanical stress. To solve this problem, complete strain gauge systems are often used such as strain sensors, torque sensors or force transducers such as shear force transducers, load cells, pressure load cells, force measuring bolts or tension and compression force sensors. As a rule, these systems are protected by accommodating them in their own housing to be attached to existing components.
It has also been tried to use strain gauges on a sliding step to directly evaluate the curvature of the lateral arms of the tread plate. However, no reliable readings could be obtained with strain gauges that were bolted onto the insides of both lateral arms, since even the sequence in which the fastening bolts were tightened had a great effect on the measuring signal. In addition, with a load, the zero point shifted to such an extent that a reliable evaluation was almost impossible.